The problem which our invention addresses is the efficient conversion of thermal energy to electrical energy. Thermophotovoltaic systems for conversion of thermal to electrical energy are well known. These are characterized by moderately inefficient conversion of thermal energy to light energy under optimal conditions, a low band gap photovoltaic device to convert the broadest range of light energy, and a need for cooling the photovoltaic device to prevent the low efficiency conversion of light to electrical energy from even further degradation resulting from a decrease in open circuit voltage associated with increasing temperatures. These characteristics, and the associated problems of prior art thermophotovoltaic (TPV) systems, are best illustrated by a brief examination of existing TPV systems.
A typical power input, or incident radiation, to photovoltaic devices in TPV systems is broad band radiation composed of wavelengths generally in the 1-2 micron range, corresponding to energies in the 0.6 to 1.2 electron volts range. In order for the photovoltaic device to capture as much energy in the broad band radiation as is possible, the band gap must match the lower energies of the radiation, that is, devices with a band gap on the order of no more than about 0.6 electron volts are desirable. This band gap ensures that all energy in the light spectrum is captured, but photon capture is not equally productive for all photons. Thus, that portion of the incident radiation having an energy close to the band gap of the photovoltaic device will effectively promote valence electrons into the conduction band; this leads to very efficient conversion of photons to current carriers. However, light energy substantially in excess of the band gap will promote valence electrons to energy states above the conduction band. These electrons undergo vibrational and rotational decay until they, too, achieve the energy of the conduction band, when as current carriers the electrons can effect conversion to electrical energy in an external circuit. But note that such electrons have lost much energy via vibrational and rotational decay, an energy loss manifested as thermal energy captured by the photovoltaic device itself which leads to a substantial increase in the device's operating temperature. Thus it is clear that the efficiency of energy conversion by a TPV device inherently decreases with an increase in the amount of thermal energy produced by incident radiation with an energy substantially in excess of the band gap.
But both the band gap and the open circuit voltage of the photovoltaic device decrease with increasing temperature. A consequence of decreasing the band gap is that still more of the total incident radiation will be converted to thermal energy because more photons will have energy in excess of that needed to move a valence electron into the conduction band. A consequence of decreasing the open circuit voltage, V.sub.oc, is a lower efficiency of power conversion. This arises because in general the fill factor of a photovoltaic device, i.e., the highest value of the product of current times voltage divided by the product I.sub.sc V.sub.oc, where I.sub.sc is the short circuit current, decreases as the open circuit voltage decreases. Thus, for photovoltaic devices with a band gap of 0.6 eV and a V.sub.oc .about.0.4 volts there is a necessity to cool the photovoltaic device for the sake of maintaining power conversion efficiency.
Where TPV conversion is desired in space, the cooling of photovoltaic devices occurs only via radiation and requires rather large, bulky radiative fins, which in turn exacts weight and volume penalties on the space vehicle. When devices operate at higher temperatures, the size of the radiator drops dramatically following a T.sup.4 power law. Thus, to recapitulate concisely, there is a cascade of undesirable effects arising from broad band radiation having photons with "excess" energy (energy greater than the band gap) which is converted, in large part, to thermal energy, lowering V.sub.oc and thereby decreasing the efficiency of power conversion.
Since many of the undesirable characteristics of present TPV systems arises from a broad radiation energy band impinging on the photovoltaic device, it follows that a narrow radiation energy band could confer important benefits. It also ought to be clear that a higher V.sub.oc when used in prior art TPV devices would maximize conversion efficiency of light to electrical energy. It also follows that if the energy spectrum of the radiation source would closely match the band gap of the photovoltaic device at operating temperatures, there would be minimal energy loss--and consequently maximal energy conversion efficiency--via vibrational and rotational decay from electronic states above the conduction band. Since photovoltaic devices can be made with band gaps over the wide range of 0.3-3 eV, the problem is "merely" one of converting broad band light radiation arising from thermal excitation into a narrow band energy spectrum matching the band gap of a photovoltaic device in the 0.3-3 electron volt range where the device has an open circuit voltage at least as high as the prior art devices. And since the efficiency of power conversion increases with increasing V.sub.oc, it follows that photovoltaic devices with a band gap greater than 0.6 eV are the more desirable devices.
We have achieved this goal by using selective infrared emitters as a means of converting thermal radiation incident on the emitters into narrow band infrared radiation with a peak energy on the order of about 1.2 electron volts. Using photovoltaic devices with a band gap in the 0.75-1.4 eV range, virtually all of the photons from selective infrared emitters can be effectively used to place electrons just into the conduction band of the photovoltaic device. Furthermore, since our higher band gap devices have both a higher open circuit voltage and an inherently higher fill factor, higher conversion efficiencies of light to electrical energy are also achieved. We also shall demonstrate that the photovoltaic device can be operated at temperatures in the 100-300.degree. C. range with additional benefits.